Is Flatten Nested List Iterator easy or hard?

Flatten Nested List Iterator is considered a Medium difficulty problem. The traversal logic itself is straightforward, but designing a lazy iterator that correctly handles nested lists requires careful stack management.

Flatten Nested List Iterator Python/Java solution

Python and Java solutions typically implement a NestedIterator class with next() and hasNext() methods. The recursive solution flattens the list during initialization, while the optimal solution maintains a stack that expands nested lists during iteration.

How to solve Flatten Nested List Iterator in O(n)?

Traverse the nested structure using depth-first traversal. Either recursively flatten all integers into an array first, or maintain a stack that expands nested lists dynamically. Each integer is visited once, giving O(n) total time complexity.

What is the best approach for Flatten Nested List Iterator?

The stack-based iterative iterator is generally the best approach. It processes nested lists lazily using a stack and expands lists only when needed during hasNext(). This keeps time complexity O(n) while using O(d) space where d is the nesting depth.

Is Flatten Nested List Iterator asked at Google/Amazon/Meta?

Flatten Nested List Iterator has appeared in interviews at large tech companies including Google, Amazon, and Meta. It tests iterator design, stack usage, and depth-first traversal of nested structures.

What data structure is used in Flatten Nested List Iterator?

The most common data structure used is a stack to simulate depth-first traversal iteratively. Some solutions also rely on recursion, which implicitly uses the call stack. The problem also emphasizes iterator design patterns.

What is the time complexity of Flatten Nested List Iterator?

Both common approaches run in O(n) time, where n is the total number of integers across all nested lists. Each element is processed once either during recursive flattening or while being expanded from the stack during iteration.

Description Solution Video 10:14

341. Flatten Nested List Iterator

Medium65.6% acceptance19 companies

You are given a nested list of integers nestedList. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.

Implement the NestedIterator class:

NestedIterator(List<NestedInteger> nestedList) Initializes the iterator with the nested list nestedList.
int next() Returns the next integer in the nested list.
boolean hasNext() Returns true if there are still some integers in the nested list and false otherwise.

Your code will be tested with the following pseudocode:

initialize iterator with nestedList
res = []
while iterator.hasNext()
    append iterator.next() to the end of res
return res

If res matches the expected flattened list, then your code will be judged as correct.

Example 1:

Input: nestedList = [[1,1],2,[1,1]]
Output: [1,1,2,1,1]
Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

Example 2:

Input: nestedList = [1,[4,[6]]]
Output: [1,4,6]
Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].

Constraints:

1 <= nestedList.length <= 500
The values of the integers in the nested list is in the range [-10⁶, 10⁶].

Asked at 19 companies

Adobe Twitter Mixpanel Hbo Openai WarnerMedia Tesla Yahoo Aurora Apple LinkedIn Walmart Labs

+7 more

Meta D. E. Shaw Airbnb

Google Amazon Bloomberg Microsoft

Topics

10:14

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LanguageLoading editor…

1# """
2# This is the interface that allows for creating nested lists.
3# You should not implement it, or speculate about its implementation
4# """
5#class NestedInteger:
6#    def isInteger(self) -> bool:
7#        """
8#        @return True if this NestedInteger holds a single integer, rather than a nested list.
9#        """
10#
11#    def getInteger(self) -> int:
12#        """
13#        @return the single integer that this NestedInteger holds, if it holds a single integer
14#        Return None if this NestedInteger holds a nested list
15#        """
16#
17#    def getList(self) -> [NestedInteger]:
18#        """
19#        @return the nested list that this NestedInteger holds, if it holds a nested list
20#        Return None if this NestedInteger holds a single integer
21#        """
22 
23class NestedIterator:
24    def __init__(self, nestedList: [NestedInteger]):
25        
26    
27    def next(self) -> int:
28        
29    
30    def hasNext(self) -> bool:
31         
32 
33# Your NestedIterator object will be instantiated and called as such:
34# i, v = NestedIterator(nestedList), []
35# while i.hasNext(): v.append(i.next())

Starter codePython 3Spaces: 4

Examples

Case 1Case 2

Input

[[1,1],2,[1,1]]

Input

[1,[4,[6]]]

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